Can't log in with boost activated (File does not exist: /user)

As soon as I paste the code generated by boost module to .htaccess, I can't log in anymore. It's like a refresh, redirecting me to /user again. The apache error log prints this message:

[error] File does not exist: /var/www/example.com/drupal/user, referer: https://dev.example.com

As soon as I comment out the Boost part in .htaccess, login works just fine. It happens both with and without SSL and it doesn't matter if Boost is configured to cache https pages or not.